The field of the present invention is polyamide powders for the coating of metals.
The invention is particularly concerned with methods for the production of pulverulent coating compositions pigmented with titanium dioxide based on polyamides having at least ten aliphatically bound carbon atoms per carbonamide group by the precipitation method. Copolyamides or a mixture of homo- and copolyamides containing at least 70% of the stated components are also useful.
The state of the prior art of producing polyamide powders may be ascertained by reference to U.S. Pat. Nos. 3,476,711; 3,900,607; 3,927,141; 3,966,838; 4,143,025; 4,195,162; 4,273,919; and 4,334,056; British Pat. Nos. 535,138; 688,771; and 1,392,949; and the Kirk-Othmer "Encyclopedia of Chemical Technology", 2nd Ed., Vol. 16 (1968), under the section "Polyamide (plastics", pages 88-105, particularly page 92 - polylauryllactam (nylon-12), and polyundecanamide (nylon-11), page 101 Solution Processes, and Powder Processing, pages 101-102, the disclosures of which are incorporated herein by reference.
The flame spraying and fluidized bed coating of nylon on a metal base is disclosed in U.S. Pat. No. 3,203,822.
The apparatus for determining the nucleating effect of the titanium dioxide pigment used in the present invention is known from Chem. Ing. Technik 51, (1979), No. 8, page 823, the disclosure of which is incorporated herein by reference.
The use of pulverulent coating compositions based on polyamides to prepare varnish-type metal coatings is known. The coating is implemented by the melt film method, that is, by the fluidized bed method, the flame spray procedure, or by the electrostatic coating process. The polyamide powders are manufactured by precipitating the polyamide from solutions as disclosed in British Pat. No. 688,771, or by grinding the polyamide granulate, preferably at low temperatures and in an inert gas atmosphere as disclosed in U.S. Pat. No. 4,273,919.
The precipitation method of British Pat. No. 688,771 describes the precipitation of polyamide powders from ethanol by cooling the hot polyamide solution externally or by letting it stand. It is only by cooling or letting the hot polyamide solution stand that powders with a high proportion of fine grains is obtained and these fine particles result in dust generation and thick smoking in the fluidized bed. Lastly, such a procedure causes undesired agglomerations which subsequently must be abraded or ground into the required powder size. Moreover, such precipitation methods do not offer reproducible batches, that is, they result in differing production batches which vary in particle size, molecular weight and bulk density.
It is furthermore known to manufacture polyamide powders by grinding polyamides of low molecular weights and by bringing the powders so obtained thereupon to the desired viscosity by heating them to temperatures below the melting point as disclosed in British Pat. No. 535,138, and U.S. Pat. No. 3,476,711.
Polylauryllactam powders also are prepared by this method and are also used in a known procedure for coating as disclosed in Chem. Ind., November 1968, pp 783-791, and Modern Plastics, February 1966, pp. 153-156. Because the polylauryllactam powders do not always meet the requirements of high elasticity, resistance to alkaline aqueous solutions and frequently tend to emit thick smokes, especially during processing, the most diverse improvements have been used. Illustrations of these diverse uses are plasticizer-containing polylauryllactam powders as disclosed in U.S. Pat. No. 3,900,607, those made from a mixture of homopolylauryllactam and lauryllactam containing copolyamides as disclosed in British Pat. No. 1,392,949, those containing polyamides with N-alkoxymethyl groups aside from acidically reacting catalysts as disclosed in U.S. Pat. No. 3,966,838, or mixtures of polyamides having 8 to 11 aliphatically bound carbon atoms per carbonamide group, aminoplasts bearing alkoxyalkyl groups and acidically reacting catalysts as disclosed in U.S. Pat. No 3,927,141. In specific instances these powders offer good properties; however, they fail to fully meet all requirements.
An improved method is described in U.S. Pat. Nos. 4,143,025 and 4,195,162. This method is still unsatisfactory in that a grinding procedure is employed to produce pigment free powders and, in order to manufacture pigmented powders, the precipitation method must be used. Lastly, this method is based on the assumed use of polylauryllactam granulate starting materials prepared exclusively by hydrolytic polymerization in the presence of specific amounts of phosphoric acid.
U.S. Pat. No. 4,334,056 discloses a substantially improved procedure comprising a method of producing polyamide powder from polyamide having a relative viscosity of 1.4 to 1.8 as measured in 0.5% meta-cresol solution at 25.degree. C. and at least 70% of said polyamide consisting of polyamide with at least ten aliphatically bound carbon atoms per carbonamide group, comprising:
(a) dissolving said polyamide in at least twice the amount by weight of ethanol in a closed vessel at a temperature between about 130.degree. to 150.degree. C. to form a solution of said polyamide;
(b) cooling said solution to a precipitation temperature between about 100.degree. and 125.degree. C. and ceasing said cooling at said precipitation temperature;
(c) precipitating said polyamide powder from said cooled solution of (b) polytropically with agitation and under an inert gas atmosphere; and
(d) separating said precipitated polyamide powders of (c) from said ethanol.
Still, this procedure needs, on the one hand, improvement especially with regard to shortening the precipitation. Such an improvement is proposed in copending U.S. patent application Ser. No. 840,819 of MUMCU ET AL. On the other hand, pigmented pulverulent coating compositions still suffer from drawbacks deserving improvement.
Pigment containing powders, especially those containing titanium dioxide, frequently evince low bulk densities of 300 to 400 grams/liter. However, the bulk density of coating powders, especially those to be used in the fluidized bed process, should be above 400 grams/liter, in particular in the range from 500 to 700 grams/liter.
Such fluidized bed powders as a rule evince a grain size between 40 and 250 microns. It is known that powders having a low bulk density entail the drawback of excess powder running very poorly off the coated object, or not at all, whereby the coating is irregular. This shortcoming cannot be remedied by adding antistatic compositions, quite aside from the fact that this would introduce other drawbacks. As a rule this low bulk density is caused by the coating powders having a porous structure.
Lastly, the known precipitation methods result in a grain size distribution which changes during the drying process. This is especially the case where there is a shift toward grain sizes that are too small, in which case the proportion of the finest grains, i.e., those less than ten microns, becomes enlarged. This proportion of dust makes subsequent sifting mandatory and inherently an additional complex processing step is required.